In video display systems utilizing a cathode ray tube (CRT) as the display device, a video output or driver stage is employed to amplify relatively low level video signals so as to produce relatively high level video signals suitable for direct application to the CRT. The high level video output signals typically are representative of red, green and blue components of the reproduced image, and are coupled to respective cathodes of the CRT. High supply voltages are coupled to the anode and various grids of the CRT from respective high voltage power supplies. As the average level of the reproduced image tends toward white, the currents drawn from the high voltage supplies increase, and in extreme cases may cause one or more of the high voltages to decrease in amplitude or "slump". The latter may cause the cut-off level of the CRT to increase resulting in the loss of detail in dark portions of the image.
A solution to the above described problem is to employ high voltage power supplies with sufficient current handling capabilities. However, such supplies are expensive. A more cost effective solution is illustrated in FIG. 1.
The final stages of the red (R), green (G) and blue (B) channels of a video display system are shown in FIG. 1. Since the three channels are substantially identical, only the red channel will be described in detail. The relatively low level red video signal which is developed at the output of pre-amplifier 10R, is amplified by driver 12R, and the resultant relatively high level red video signal is coupled through a resistor 14R to a respective cathode 16R of a CRT 18. A common first grid G1 of cathode 18 receives a supply voltage from a G1 voltage supply 20. A common second or screen grid G2, a common first focus grid F1, and a common second focus grid F2 receive respective relatively higher voltages from respective outputs of a high voltage section 22. A very high voltage is coupled to anode A of CRT 18 from another output of high voltage section 22.
Driver 12R comprises NPN transistors Q1 and Q2 connected in a cascode amplifier configuration. The output of preamplifier 10R is coupled to the base of transistor Q1. A reference voltage VREF is coupled to the emitter of transistor through an emitter resistor RE. A bias voltage +VCC is coupled to the base of transistor Q2. The collector of transistor Q2 is coupled to a source of supply voltage B+ through a load, shown simply as a resistor RL, and a resistor RS, the purpose of which will be described below. The collector of transistor Q2 is also coupled through a resistor 14R to cathode 16R. A filter capacitor CS is coupled between the junction of load RL and resistor RS and signal ground. The cutoff level of CRT 18 is a function of reference voltage VREF and bias voltage VCC. The network including resistor RS and capacitor CS is provided to reduce the possibility of the loss of details in dark portions of the image during high average picture level images, as will now be explained.
Decreasing cathode voltage corresponds to increasing the "whiteness" of the image and increasing cathode voltage corresponds to increasing the "blackness" of the image. The beam current drawn by CRT 18 from high voltage section 22 increases as the average picture level increases toward white. The voltages supplied to anode and various grids of CRT 18 by high voltage network 22 tend to decrease at relatively high beam currents. If the cathode voltage were to remain constant, this would cause the cutoff level of CRT 18 to increase and cause dark areas of the image to get darker, with a resulting loss of detail. However, resistor RS produces a voltage drop which decreases the cathode voltage as the beam current increases. The voltage drop developed across resistor RS compensates for the reduction in amplitude (or "slump") of the high voltages supplied by high voltage network 22 and therefore inhibits the loss of detail in dark areas as the average picture level increases. Thus, resistor RS can be thought of as providing a "black tracking" function which operates by reducing the effective supply voltage of B+ voltage supply 24 with increases of beam current. Capacitor CS is required to remove the AC component at the effective supply voltage terminal at the junction of resistors RS and RL.
While the network including resistor RS and capacitor CS performs its intended function satisfactorily, it does have certain drawbacks. The value of resistor RS has to be relatively large for the intended compensation function. However, as a consequence, the "head-room" of the driver, i.e., the amplitude range of the output signal of the driver, is restricted. In addition, since the B+ voltage is relatively high (e.g., +220 volts), the physical size of capacitor CS is large, even for small capacitance values.